Miliband said he was “deeply concerned” about staff being detained, stressing his confidence that they had not engaged in illegal activities.

“We have noted the remarks by Ayatollah (Ahmad) Jannati suggesting that some of our local staff in Iran may face trial,” Miliband said in a statement.

“We are urgently seeking clarification from the appropriate Iranian authorities. I intend to speak to Foreign Minister (Manouchehr) Mottaki.

“We are confident that our staff have not engaged in any improper or illegal behaviour. We remain deeply concerned about the two members of our staff who remain in detention in Iran.”

Iranian state television has said only one remains in custody.

Miliband said he welcomed the decisions made Thursday by European Union countries to summon Iranian ambassadors in protest.

An EU diplomat said: “We can confirm that one of the actions agreed at yesterday’s political directors’ meeting was that EU members should as soon as possible summon the Iranian ambassadors across the EU to demand the immediate release of embassy personnel.”

Jannati, the head of Iran’s powerful Guardians Council, said at Friday prayers that some local British embassy staff would be put on trial for their alleged role in stoking post-election violence.

“In these incidents, their embassy had a presence, some people were arrested. Naturally they will be put on trial, they have made confessions,” he said in Tehran.

Iran has accused Britain of manipulating the violent protests following the disputed election of Iranian President Mahmoud Ahmadinejad, allegations rejected by London.

Tehran has already expelled two British diplomats from Iran in actions reciprocated by London.

Following Iran’s announcement of a trial, a Foreign Office spokeswoman told AFP they were now “following up with the Iranian authorities”.

The spokeswoman said: “Allegations that our staff were fomenting unrest are wholly without foundation.”